Luther Douglas S.
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ArticleThe Cascadia Initiative : a sea change In seismological studies of subduction zones(The Oceanography Society, 2014-06) Toomey, Douglas R. ; Allen, Richard M. ; Barclay, Andrew H. ; Bell, Samuel W. ; Bromirski, Peter D. ; Carlson, Richard L. ; Chen, Xiaowei ; Collins, John A. ; Dziak, Robert P. ; Evers, Brent ; Forsyth, Donald W. ; Gerstoft, Peter ; Hooft, Emilie E. E. ; Livelybrooks, Dean ; Lodewyk, Jessica A. ; Luther, Douglas S. ; McGuire, Jeffrey J. ; Schwartz, Susan Y. ; Tolstoy, Maya ; Trehu, Anne M. ; Weirathmueller, Michelle ; Wilcock, William S. D.Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology—shielded ocean bottom seismometers—is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.
ArticleHigh-Q spectral peaks and nonstationarity in the deep ocean infragravity wave band: Tidal harmonics and solar normal modes(American Geophysical Union, 2019-02-20) Chave, Alan D. ; Luther, Douglas S. ; Thomson, David J.Infragravity waves have received the least study of any class of waves in the deep ocean. This paper analyzes a 389‐day‐long deep ocean pressure record from the Hawaii Ocean Mixing Experiment for the presence of narrowband (≲2 μHz) components and nonstationarity over 400–4,000 μHz using a combination of fitting a mixture noncentral/central χ2 model to spectral estimates, high‐resolution multitaper spectral estimation, and computation of the offset coherence between distinct frequencies for a given data segment. In the frequency band 400–1,000 μHz there is a noncentral fraction of 0.67 ± 0.07 that decreases with increasing frequency. Evidence is presented for the presence of tidal harmonics in the data over the 400‐ to 1,400‐μHz bands. Above ~2,000 μHz the noncentral fraction rises with frequency, comprising about one third of the spectral estimates over 3,000–4,000 μHz. The power spectrum exhibits frequent narrowband peaks at 6–11 standard deviations above the noise level. The widths of the peaks correspond to a Q of at least 1,000, vastly exceeding that of any oceanic or atmospheric process. The offset coherence shows that the spectral peaks have substantial (p = 0.99–0.9999) interfrequency correlation, both locally and between distinct peaks within a given analysis band. Many of the peak frequencies correspond to the known values for solar pressure modes that have previously been observed in solar wind and terrestrial data, while others are the result of nonstationarity that distributes power across frequency. Overall, this paper documents the existence of two previously unrecognized sources of infragravity wave variability in the deep ocean.
ThesisObservations of long period waves in the tropical oceans and atmosphere(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1980-02) Luther, Douglas S.The existence of resonant, baroclinic, equatorially-trapped inertia-gravity waves (discovered by Wunsch and Gill (1976)) is confirmed in the mid-Pacific by spectral analysis of long sea level records. The energy of the low-mode inertia-gravity waves is found to decrease toward the meridional boundaries. A simple spectral model, acknowledging the dispersive characteristics of the equatorial waves, adequately reproduces the observed mid-Pacific sea level spectra in the 1-6 day band. Model spectra computed at latitudes outside the equatorial waveguide of the gravest meridional modes suggest the presence of "inertial" peaks in several observed sea level spectra. Resonant, low-mode inertia-gravity waves may also exist in the Indian Ocean. Sea level fluctuations along the Pacific equator are found to have Kelvin wave characteristics in the 35-80 day band, and, in particular, propagation from the western Pacific to the coast of South America is observed. The Kelvin waves are atmospherically-forced in the central- western Pacific and have a computed equivalent depth corresponding to the first-baroc1inic mode. Outside of the equatorial mid-Pacific, a non-static ocean response to air pressure in the 4-6 day band is dominated by a basin-wide, barotropic, planetary mode. The low Q of this mode suggests that the ocean is viscous with respect to large-scale barotropic oscillations. The dynamical components of the observed long-period tides have been isolated for the first time using the "self-consistent" equilibrium tide of Agnew and Farrell (1978). The tides are slightly non-equilibrium with large horizontal scales. The relatively short-scale Rossby modes predicted by Wunsch (1967) are not observed, perhaps because of the poor spatial coverage of the dataset. Considering the low Q of the 4-6 day planetary basin mode, it is suggested that the long-period tides are frictionally-controlled. The 4- and 5-day equatorial inertia-gravity waves, the 35-80 day Kelvin waves and the 4-6 day planetary basin mode are clearly atmospherically forced, and, perhaps surprisingly, they are forced by atmospheric waves that have similar horizontal structures, i.e., 4-5 day Rossby-gravity waves, 40-50 day Kelvin waves and a 5-day global barotropic mode. The surface expressions of these atmospheric waves are determined in order to understand the nature of the oceanic response, e.g., resonant or forced. Much of the information about the surface atmospheric fields that has been collected, including frequency-wavenumber descriptions, awaits an accurate model of the coupling between wind stress and internal ocean waves.